Star-forming Disk Galaxies Research Articles

North Ecliptic Pole Wide Field Survey of AKARI: Survey Strategy and Data Characteristics

Abstract We present the survey strategy and the data characteristics of the North Ecliptic Pole (NEP) Wide Survey of AKARI. The survey was carried out for about one year starting from 2006 May with 9 passbands from 2.5 to 24 $\mu$m and the areal coverage of about 5.8 degree$^2$ centered on NEP. The survey depth reaches to 21.8 AB magnitude near infrared (NIR) bands, and $\sim$18.6 AB magnitude at the mid infrared (MIR) bands such as 15 and 18 $\mu$m. The total number of sources detected in this survey is about 104000, with more sources in NIR than in the MIR. We have cross matched infrared sources with optically identified sources in CFHT imaging survey which covered about 2 degree$^2$ within NEP-Wide survey region in order to characterize the nature of infrared sources. The majority of the MIR sources at 15 and 18 $\mu$m bands are found to be star forming disk galaxies, with smaller fraction of early type galaxies and AGNs. We found that a large fraction (60%–80%) of bright sources in 9 and 11 $\mu$m is stars while stellar fraction decreases toward fainter sources. We present the histograms of the sources at MIR bands at 9, 11, 15, and 18 $\mu$m. The number of sources per magnitude thus varies as $m^{0.6}$ for longer wavelength sources while shorter wavelength sources show steeper variation with $m$, where $m$ is the AB magnitude.

The Evolution of Galaxy Mergers and Morphology atz< 1.2 in the Extended Groth Strip

We present the quantitative rest-frame B morphological evolution and galaxy merger fractions at 0.2 < z < 1.2 as observed by the All-wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficent and M_20 to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4 L_B^*, assuming pure luminosity evolution of 1.3 M_B per unit redshift. We find that the merger fraction remains roughly constant at 10 +/- 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa increases from 21+/- 3% at z ~ 1.1 to 44 +/- 9% at z ~ 0.3, while the fraction of Sb-Ir decreases from 64 +/- 6% at z ~ 1.1 to 47 +/- 9% at z ~ 0.3. The majority of z < 1.2 Spitzer MIPS 24 micron sources with L(IR) > 10^11 L_sun are disk galaxies, and only ~ 15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ~ 1.1, while E/S0/Sa makeup over 90% of the red sequence at z ~ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the galaxy merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star-formation rate density since z ~ 1 is a result of declining star-formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ~ 1.2.

Large‐Scale Gravitational Instability and Star Formation in the Large Magellanic Cloud

Large-scale star formation in disk galaxies is hypothesized to be driven by global gravitational instability. The observed gas surface density is commonly used to compute the strength of gravitational instability, but according to this criterion star formation often appears to occur in gravitationally stable regions. One possible reason is that the stellar contribution to the instability has been neglected. We have examined the gravitational instability of the Large Magellanic Cloud (LMC) considering the gas alone, and considering the combination of collisional gas and collisionless stars. We compare the gravitationally unstable regions with the on-going star formation revealed by Spitzer observations of young stellar objects. Although only 62% of the massive young stellar object candidates are in regions where the gas alone is unstable, some 85% lie in regions unstable due to the combination of gas and stars. The combined stability analysis better describes where star formation occurs. In agreement with other observations and numerical models, a small fraction of the star formation occurs in regions with gravitational stability parameter Q > 1. We further measure the dependence of the star formation timescale on the strength of gravitational instability, and quantitatively compare it to the exponential dependence expected from numerical simulations.

Molecular cloud regulated star formation in galaxies

We describe a numerical implementation of star formation in disc galaxies, in which the conversion of cooling gas to stars in the multiphase interstellar medium is governed by the rate at which molecular clouds are formed and destroyed. In the model, clouds form from thermally unstable ambient gas and get destroyed by feedback from massive stars and thermal conduction. Feedback in the ambient phase cycles gas into a hot galactic fountain or wind. We model the ambient gas hydrodynamically using smoothed particle hydrodynamics. However, we cannot resolve the Jeans mass in the cold and dense molecular gas and, therefore, represent the cloud phase with ballistic particles that coagulate when colliding. We show that this naturally produces a multiphase medium with cold clouds, a warm disc, hot supernova bubbles and a hot, tenuous halo. Our implementation of this model is based on the gadget N-body code. We illustrate the model by evolving an isolated Milky Way-like galaxy and study the properties of a disc formed in a rotating spherical collapse. Many observed properties of disc galaxies are reproduced well, including the molecular cloud mass spectrum, the molecular fraction as a function of radius, the Schmidt law, the stellar density profile and the appearance of a galactic fountain.

Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by theSpitzer Space Telescope

We present ground-based optical and Spitzer Space Telescope infrared imaging observations of the interacting galaxy UGC 10214, the Tadpole galaxy (z = 0.0310), focusing on the star formation activity in the nuclear, disk, spiral arms, and tidal tail regions. The ground-based optical data set spans a wavelength range between 0.3 and 0.8 μm, the near-IR data set spans 1–2.2 μm, and the Spitzer IR data set spans 3–70 μm. The major findings of this study are that the Tadpole is actively forming stars in the main disk outside of the nucleus and in the tidal plume, with an estimated mean star formation rate of ~2–4 M⊙ yr-1. The most prominent sites of mid-IR emission define a morphology that, combined with the overall morphology of the system, suggests the interaction may belong to the rare class of off-center collisional ring systems that form both shock-induced rings of star formation and tidal plumes. In stark contrast to the disk star formation, the nuclear emission is solely powered by older stars, with little evidence for ongoing star formation at the center of the Tadpole. Extranuclear star formation accounts for >50% of the total star formation in the disk and spiral arms, featuring infrared-bright spots that exhibit strong polycyclic aromatic hydrocarbon (PAH) emission, the band strength of which is comparable to that of late-type star-forming disk galaxies. The tidal tail, which extends 2' (~75 kpc) into the intergalactic medium, is populated by supermassive star clusters, M ~ 106 M⊙, likely triggered by the galaxy-galaxy interaction that has distorted UGC 10214 into its current tadpole shape. The Tadpole is therefore an example of an off-nuclear or tidal-tail starburst, with several large sites of massive star formation in the disk and in the plume, including the most prominent Hubble Space Telescope–revealed cluster, J160616.85+552640.6. The clusters exhibit remarkable IR properties, including exceptionally strong 24 μm emission relative to the underlying starlight, hot dust continuum, and PAH emission, with an estimated current star formation rate of ~0.1–0.4 M⊙ yr-1, representing >10% of the total star formation in the system. We estimate the mass of the largest cluster to be ~ × 106 M⊙ based on the g'-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes in combination with an assumed mass-to-light ratio appropriate to young clusters, or large enough to be classified as a nascent dwarf galaxy or globular cluster.

Structure and star formation in disk galaxies

Structure and star formation in disk galaxies

Structure and star formation in disk galaxies

We present optical observations of a sample of 57 spiral galaxies and describe the procedures followed to reduce the data. We have obtained images in the optical B and I broad bands, as well as in Hα, with moderate spatial resolution and across wide enough fields to image the complete disks of the galaxies. In addition, we observed 55 of our sample galaxies in the R and eight in the V band, and imaged a subset through a dedicated narrow continuum filter for the Hα line. We describe the data reduction procedures we developed in the course of this work to register, combine and photometrically calibrate each set of images for an individual galaxy. We describe in some detail the procedure used to subtract the continuum emission from our Hα images. In companion papers, we describe near-infrared imaging of the galaxy sample, and present analyses of disk scale lengths, and of properties of bars, rings, and H  regions in the sample galaxies. The images described here are available for use by other researchers.

A High Spatial Resolution X‐Ray and Hα Study of Hot Gas in the Halos of Star‐forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X‐Ray Emission

We present arcsecond resolution Chandra X-ray and ground-based optical Hα imaging of a sample of 10 edge-on star-forming disk galaxies (seven starburst and three spiral galaxies), a sample that covers the full range of star formation intensity found in disk galaxies. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extraplanar diffuse X-ray emission is very similar to the extraplanar Hα filaments and arcs, on both small and large scales (scales of tens of parsecs and kiloparsecs, respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse X-ray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extraplanar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range Heff ~ 2-4 kpc. The presence of extraplanar X-ray emission is always associated with the presence of extraplanar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low-mass normal spiral galaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present, appear to play no role in powering or shaping the outflows from the starburst galaxies in this sample. The Chandra ACIS X-ray spectra of extraplanar emission from all these galaxies can be fitted with a common two-temperature spectral model with an enhanced α-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow-out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height Hg ~ 4-8 kpc. The soft thermal X-ray emission observed in the halos of the starburst galaxies is either this preexisting halo medium, which has been swept up and shock-heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case, the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.

Forming Young Bulges within Existing Disks: Statistical Evidence for External Drivers

Contrary to traditional models of galaxy formation, recent observations suggest that some bulges form within preexisting disk galaxies. Such late-epoch bulge formation within disks seems to be linked to disk gas inflow and central star formation, caused by either internal secular processes or galaxy mergers and interactions. We identify a population of galaxies likely to be experiencing active bulge growth within disks, using the criterion that the color within the half-light radius is bluer than the outer disk color. Such blue-centered galaxies make up >10% of star-forming disk galaxies within the Nearby Field Galaxy Survey, a broad survey designed to represent the natural diversity of the low-z galaxy population over a wide range of luminosities and environments. Blue-centered galaxies correlate at 99% confidence with morphological peculiarities suggestive of minor mergers and interactions. From this and other evidence, we argue that external drivers rather than internal secular processes probably account for the majority of blue-centered galaxies. We go on to discuss quantitative plausibility arguments indicating that blue-centered evolutionary phases may represent an important mode of bulge growth for most disk galaxies, leading to significant changes in bulge-to-disk ratio without destroying disks. If this view is correct, bulge growth within disks may be a natural consequence of the repeated galaxy mergers and interactions inherent in hierarchical galaxy formation.

Erratum: Structure and star formation in disc galaxies -- I. Sample selection and near-infrared imaging

Erratum: Structure and star formation in disc galaxies -- I. Sample selection and near-infrared imaging

Structure and star formation in disc galaxies - I. Sample selection and near-infrared imaging

We present near-infrared imaging of a sample of 57 relatively large, northern spiral galaxies with low inclination. After describing the selection criteria and some of the basic properties of the sample, we give a detailed description of the data collection and reduction procedures. The Ksλ= 2.2-μm images cover most of the disc for all galaxies, with a field of view of at least 4.2 arcmin. The spatial resolution is better than 1 arcsec for most images. We fit bulge and exponential disc components to radial profiles of the light distribution. We then derive the basic parameters of these components, and the bulge/disc ratio, and explore correlations of these parameters with several galaxy parameters.

Infrared Emission of Normal Galaxies from 2.5 to 12 Micron:Infrared Space ObservatorySpectra, Near‐Infrared Continuum, and Mid‐Infrared Emission Features

We present ISOPHOT spectra of the regions 2.5-4.9 μm and 5.8-11.6 μm for a sample of 45 disk galaxies from the US Infrared Space Observatory Key Project on Normal Galaxies. The galaxies were selected to span the range in global properties of normal, star-forming disk galaxies in the local universe. The spectra can be decomposed into three spectral components: (1) continuum emission from stellar photospheres, which dominates the near-infrared (NIR; 2.5-4.9 μm) spectral region; (2) a weak NIR excess continuum, which has a color temperature of ~103 K, carries a luminosity of a few percent of the total far-infrared (FIR) dust luminosity LFIR and most likely arises from the interstellar medium (ISM); and (3) the well-known broad emission features at 6.2, 7.7, 8.6, and 11.3 μm, which are generally attributed to aromatic carbon particles. These aromatic features in emission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 μm) part of the spectrum and resemble the so-called type A spectra observed in many nonstellar sources and the diffuse ISM in our own Galaxy. The few notable exceptions include NGC 4418, where a dust continuum replaces the AFEs in MIR, and NGC 1569, where the AFEs are weak and the strongest emission feature is [S IV] 10.51 μm. The relative strengths of the AFEs vary by 15%-25% among the galaxies. However, little correlation is seen between these variations and either IRAS 60 μm/100 μm flux density ratio R(60/100) or the FIR/blue luminosity ratio LFIR/LB, two widely used indicators of the current star formation activity, suggesting that the observed variations are not a consequence of the radiation field differences among the galaxies. We demonstrate that the NIR excess continuum and AFE emission are correlated, suggesting that they are produced by similar mechanisms and similar (or the same) material. On the other hand, as the current star formation activity increases, the overall strengths of the AFEs and the NIR excess continuum drop significantly with respect to that of the FIR emission from large dust grains. In particular, the summed luminosity of the AFEs falls from ~0.2 LFIR for the most IR-quiescent galaxies to ~0.1 LFIR for the most IR-active galaxies. This is likely a consequence of the preferential destruction in intense radiation fields of the small carriers responsible for the NIR/AFE emission.

A multi-phase model for simulations of galaxy formation

We discuss SPH simulations of galaxy formation which use a hybrid method to describe a two-phase structure of the star forming ISM on unresolved scales. Our modeling includes radiative cooling, heating due to a UV background, growth of cold clouds embedded in an ambient hot gas, star formation out of cloud material, feedback due to supernovae in the form of thermal heating and cloud evaporation, starbursts that can lead to galactic outflows, and metal enrichment. Our particular model for the treatment of the two-phase structure is based on a modified and extended version of the grid-based approach of Yepes et al. (1997). We discuss the properties of the feedback model and show how it stabilizes star forming disk galaxies and reduces the cosmic star formation rate to a level consistent with current observational constraints.

Cosmological smoothed particle hydrodynamics simulations: a hybrid multiphase model for star formation

We present a model for star formation and supernova feedback that describes the multi-phase structure of star forming gas on scales that are typically not resolved in cosmological simulations. Our approach includes radiative heating and cooling, the growth of cold clouds embedded in an ambient hot medium, star formation in these clouds, feedback from supernovae in the form of thermal heating and cloud evaporation, galactic winds and outflows, and metal enrichment. Implemented using SPH, our scheme is a significantly modified and extended version of the grid-based method of Yepes et al. (1997), and enables us to achieve high dynamic range in simulations of structure formation. We discuss properties of the feedback model in detail and show that it predicts a self-regulated, quiescent mode of star formation, which, in particular, stabilises the star forming gaseous layers of disk galaxies. The parameterisation of this mode can be reduced to a single free quantity which determines the overall timescale for star formation. We fix this parameter to match the observed rates of star formation in local disk galaxies. When normalised in this manner, cosmological simulations nevertheless overproduce the observed cosmic abundance of stellar material. We are thus motivated to extend our feedback model to include galactic winds associated with star formation. Using small-scale simulations of individual star-forming disk galaxies, we show that these winds produce either galactic fountains or outflows, depending on the depth of the gravitational potential. Moreover, outflows from galaxies in these simulations drive chemical enrichment of the intergalactic medium, in principle accounting for the presence of metals in the Lyman alpha forest. (abridged)

Complex modeling the chemo-photometric evolution of disk galaxy

The chemical and photometric evolution of star forming disk galaxies is investigated. Numerical simulations of the complex gasdynamical flows are based on our own coding of the Chemo-Dynamical Smoothed Particle Hydrodynamical (CD-SPH) approach, which incorporates the effect of star formation. As a first application, the model is used to describe the chemical and photometric evolution of a disk galaxy like the Milky Way.

A2111: a z = 0:23 Butcher—Oemler cluster with a non-isothermal atmosphere and normal metallicity

We report results from an X-ray study of the Abell 2111 galaxy cluster using the Advanced Satellite for Astrophysics and Cosmology (ASCA) and the ROSAT Position Sensitive Proportional Counter (PSPC). By correcting for the energy-dependent point-spread function of the ASCA instruments, we have examined the temperature structure of the cluster. The core of the cluster within 3 arcmin is found to have a temperature of 6.46±0.87 keV, significantly higher than the value of 3.10±1.19 keV in the surrounding region of r=3–6 arcmin. This radially decreasing temperature structure can be parametrized by a polytropic index of γ≃ 1.45. The X-ray morphology of the cluster a pears elongated and clumpy on scales ≤1 arcmin. These results, together with earlier ROSAT and optical studies which revealed that the X-ray centroid and ellipticity of A2111 shift with spatial scale, are consistent with the hypothesis that the cluster is a dynamically young system. Most likely, the cluster has recently undergone a merger, which may also be responsible for the high fraction of blue galaxies observed in the cluster. Alternatively, the temperature structure may also be caused by the gravitational potential of the cluster. We have also measured the emission-weighted abundance of the X-ray-emitting intracluster medium as 0.25±0.14 solar. This value is similar to those of nearby clusters that do not show a large blue galaxy fraction, indicating that star formation in disc galaxies and subsequent loss to the medium do not drastically alter the average abundance of a cluster. This is consistent with recent results, which indicate that cluster abundances have remained constant since at least z∼0.3.

Gravitational instability and star formation in disc galaxies

We present a general star formation law in which the star formation rate depends upon the efficiency α and the time-scale τ of star formation, and the gas component σg of the surface mass density. The current nominal Schmidt exponent nS for our model is 2<nS<3. Based on a gravitational instability parameter QA and another dimensionless parameter fP=(P/Gcσ2)1/2, where P = pressure and σc = column density of molecular clouds, we suggest a general equation for the star formation rate which depends upon the relative contributions of the two parameters for various physical circumstances. We find that QA turns out to be a better parameter for the star formation scenario than does the Toomre Q-parameter. The star formation rate in the solar neighbourhood is found to be in good agreement with values inferred from previous studies. In the closed box approximation model, we obtain a relationship between the metallicity of the gas and the efficiency of star formation. Our model calculations of metallicity in the solar neighbourhood agree with earlier estimates. We conclude that the metallicity dispersion for stars of the same age may result from a change in the efficiency with which different stars in the sample were processed. For no significant change of metallicity with age, we suggest that all stars in the sample were born with similar efficiencies.

Optical Structure and Colors of Faint Compact Narrow Emission Line Galaxies

We present the results of Hubble Space Telescope Wide Field Planetary Camera imaging in V606 and I814 of five compact narrow emission line galaxies (CNELGs) and four slightly more extended faint blue galaxies (FBGs) with 20.3 ≤ B ≤ 22.4 and redshifts z ~ 0.22-0.66. Half-light radii are measured to span from 014 to 067, thus yielding small sizes of Re ~ 1-5 kpc (H0=50 km s-1 Mpc-1, q0=0.1). The new scale lengths, in combination with previous Keck measurements of velocity widths, continue to confirm that CNELGs, though quite luminous (MB~-21), have masses comparable to dwarf galaxies (i.e., M/M☉ 1010). Their low mass-to-light ratios are consistent with values corresponding to very early phases of a major burst of star formation. The spatial contour maps and surface brightness profiles of CNELGs reveal the presence of high surface brightness knots surrounded by a diffuse exponential-like component and sometimes accompanied by fans and tails. Since the knots tend to also have bluer colors, they are identified as the location of the current starburst. The diffuse component is interpreted to be an older underlying population. With the current burst estimated from the rest-frame (B - V) colors and mass-to-light ratios to involve typically ~10% of the galaxy mass, CNELGs are predicted to fade by 4 mag to reach the low luminosities and surface brightnesses of bright spheroidal galaxies, such as NGC 205. These new results provide further support for our previous suggestion that these distant, low-mass, starburst galaxies may be the progenitors of today's bright spheroidals. In contrast, although the four FBGs have similar colors and mass-to-light ratios to CNELGs, they will most likely not evolve into spheroidals but will resemble small, star-forming disk galaxies.

Star Formation in Disk Galaxies Driven by Primordial H[TINF]2[/TINF

We show that gaseous H I disks of primordial composition irradiated by an external radiation field can develop a multiphase medium with temperatures between 102 and 104 K as a result of the formation of molecular hydrogen. For a given H I column density there is a critical value of the radiation field below which only the cold H I phase can exist. Because of a time-decreasing quasar background, the gas starts cooling slowly after recombination until the lowest stable temperature in the warm phase is reached at a critical redshift z = zcr. Below this redshift the formation of molecular hydrogen promotes a rapid transition toward the cold H I phase. We find that disks of protogalaxies with 1020 NH I 1021 cm-2 are gravitationally stable at T ~ 104 K and can start their star formation history only at z zcr ~ 2, after the gas in the central portion of the disk has cooled to temperatures T 200 K. Such a delayed starbust phase in galaxies of low gas surface density and low dynamical mass can disrupt the disks and cause them to fade away. These objects could contribute significantly to the faint blue galaxy population.

Past and future star formation in disk galaxies

We have combined Hα and UBV measurements of 210 nearby Sa-Irr galaxies with new photometric synthesis models to reanalyze the past and future star formation timescales in disks. The integrated photoionization rates and colors of disks are best fitted by a stellar initial mass function (IMF) which is enriched in massive stars by a factor of 2-3 relative to the Scalo solar neighborhood IMF. We have used published surface photometry of spiral galaxies to analyze the star formation histories of disks independent of their bulge properties. The ratio of the current star formation rate (SFR) to the average past rate increases from of order 0.01 in Sa galaxies to 1 in Sc-Irr disks. This confirms that the pronounced change in the photometric properties of spiral galaxies along the Hubble sequence is predominantly due to changes in the star formation histories of disks, and only secondarily to changes in the bulge/disk ratio. A comparison of current SFRs and gas masses of the sample yields median timescales for gas consumption of ~3 Gyr, in the absence of stellar recycling. However, a proper time-dependent treatment of the gas return from stars shows that recycling extends the gas lifetimes of disks by factors of 1.5-4 for typical disk parameters. Consequently the current SFRs in many (but not all) disks can be sustained for periods comparable to the Hubble time.